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Date:   Wed, 18 Jul 2018 17:34:05 +0100
From:   Daniel Thompson <daniel.thompson@linaro.org>
To:     Lee Jones <lee.jones@linaro.org>
Cc:     Marcel Ziswiler <marcel.ziswiler@toradex.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "jingoohan1@gmail.com" <jingoohan1@gmail.com>,
        "linux-pwm@vger.kernel.org" <linux-pwm@vger.kernel.org>,
        "linux-fbdev@vger.kernel.org" <linux-fbdev@vger.kernel.org>,
        "b.zolnierkie@samsung.com" <b.zolnierkie@samsung.com>,
        "thierry.reding@gmail.com" <thierry.reding@gmail.com>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        "patches@linaro.org" <patches@linaro.org>
Subject: Re: [PATCH] backlight: pwm_bl: Fix uninitialized variable
Message-ID: <20180718163405.mnebf57apzvm276w@holly.lan>
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On Wed, Jul 18, 2018 at 04:55:44PM +0100, Lee Jones wrote:
> On Wed, 18 Jul 2018, Daniel Thompson wrote:
> 
> > On Wed, Jul 18, 2018 at 02:08:53PM +0100, Lee Jones wrote:
> > > > > > > No, then we are back to the initial issue of num_steps
> > > > > > > potentially not
> > > > > > > being initialised. We really want both of_property_read_u32() to
> > > > > > > succeed AND num_steps to actually be set.
> > > > > > 
> > > > > > I also think num_steps should be pre-initialised.
> > > > 
> > > > Yes, I guess it definitely does not hurt.
> > > > 
> > > > > > Then it will only be set if of_property_read_u32() succeeds.
> > > > 
> > > > Yes, but we still need to check for both, the function not failing and
> > > > num_steps to actually be non zero.
> > > 
> > > Why?  You don't do anything differently if it fails.
> > 
> > Only if you initialize num_steps...
> > 
> > We should either initialize to zero and not worry about the return
> > code[1] or we check the return code and not worry about
> > initialization[2]. I don't think both are worthwhile.
> > 
> > Whilst initialization can fix this specific instance we generally avoid
> > overusing it since it messes up static analysis and, in this instance,
> > distance from declaration to use is >25 lines, hence current patchset.
> > 
> > 
> > Daniel.
> > 
> > 
> > [1] https://lkml.org/lkml/2018/7/16/399
> > [2] https://lkml.org/lkml/2018/7/16/1042
> > 
> > Or...
> > 
> > We check the return code and leave number
> > 
> > num_steps is uninitialized and stack allocated so it only has a valid
> > value if of_property_read_u32() succeeds.
> > 
> > We can (and I originally did) fix the bug by initializing num_steps to 0
> > but its quite some distance between declaration and use so I accepted
> > Marcel's counter proposal to check the return code instead.
> 
> Only checking the return value of of_property_read_u32() is also
> suitable.

I did think about that case... I concluded that it isn't wrong for a
DT to set to this property to 0 (effectively meaning "no interpolated
steps please").

If we take the branch when num_steps is zero we get a bunch of pointless
housekeeping that amounts to no more than an extremely elaborate
malloc/memcpy/free.


Daniel.